Ridge approximation for thin nematic polymer networks

Nematic polymer networks (NPNs) are nematic elastomers within which the nematic director is enslaved to the elastic deformation. The elastic free energy of a NPN sheet of thickness h has both stretching and bending components (the former scaling like h, the latter scaling like h3). NPN sheets bear a director field m imprinted in them (usually, uniformly throughout their thickness); they can be activated by changing the nematic order (e.g., by illumination or heating). This paper illustrates an attempt to compute the bending energy of a NPN sheet and to show which role it can play in determining the activated shape. Our approach is approximate: the activated surface consists of flat sectors connected by ridges, where the unit normal jumps and the bending energy is concentrated. By increasing the number of ridges, we should get closer to the real situation, where the activated surface is smooth and the bending energy is distributed on it. The method is applied to a disk with imprinted a spiraling planar hedgehog. It is shown that upon activation the disk, like a tiny hand, is able to grab a rigid lamina.